1. Field of the Invention
This invention relates generally to electric motor control, and more particularly, to a method and apparatus for three-phase motor control using a microcontroller having two pulse-width modulators.
2. Description of Related Art
An alternating current ("AC") is a current that is alternately positive and negative at regular intervals. One complete segment of positive and negative values is called a cycle, and every cycle defines 2.pi. radians, or 360.degree. electrical. In other words, each electrical cycle is divided into 360 equal parts. In this disclosure, the electrical cycle will be discussed in terms of electrical degrees for the sake of simplicity.
Many types of devices, such as electric motors, require electrical power to Is operate, and the most common types of electric motors are powered using AC voltages. AC motors may be single-phase or polyphase machines. An example of a polyphase AC motor is a three-phase induction motor, in which three-phase sinusoidal AC voltages are applied to the motor's windings to create a rotating magnetic field. AC induction motors are popular for several reasons, including high robustness, reliability, low price and high efficiency. In such three-phase AC systems, the three AC wave forms are 120.degree. out of phase. In other words, the peaks of the respective AC wave forms are 120.degree. apart.
Power conversion systems are commonly used to provide single phase or multiphase AC power to machines requiring electrical energy. An example of such a power conversion system is a DC-to-AC inverter, which typically includes inverter switching devices connected in a bridge formation between the DC bus lines and output terminals of the power conversion system. The inverter switching devices are controlled to connect the power on the DC bus lines to the system output terminals in a desired pattern such that AC output signals having the desired frequency and amplitude are synthesized from the DC power on the DC bus lines. Various modulation strategies may be employed for controlling the inverter switching devices, including sine wave pulse-width modulation ("PWM").
PWM consists of a sequence of pulses at a "carrier" frequency much higher than that of the waveshape to be synthesized. The duration of each pulse varies according to the instantaneous magnitude of the synthesized wave. The "duty cycle" or "duty ratio" is the ratio of the pulse width to the total PWM period (reciprocal of carrier frequency). The average voltage seen at the output of a PWM switched DC bus is equal to the duty ratio multiplied by the DC bus voltage. Thus, if the duty ratio varies with time according to a sine function, the output voltage, appropriately low-pass filtered to reject the carrier frequency, will be the same sine function. Three-phase PWM requires three such switches.
FIG. 1 is a block diagram illustrating a prior art three-phase output bridge of a DC-to-AC inverter. This is a popular power topology for driving an AC induction motor. Each of the three half-bridges are pulse-width modulated independently, and a three-phase AC sine wave is applied to the three-phase motor. The output bridge 10 includes three legs 12, with a corresponding PWM generator 14 coupled to a switch driver 15 of each inverter leg 12 to provide a PWM signal that is used to control upper and lower switches 16, 18. The upper and lower switches 16, 18 of each inverter leg 12 are controlled by the switch driver 15 to couple output terminals 20 either to a positive bus 22 or a negative bus 24 to provide the desired three-phase output.
Several companies offer microcontroller chips that include motor controller peripherals. As shown in FIG. 1, the generation of three-phase sine waves requires three PWM generators; however, chips having three PWM generators are often more expensive than chips having fewer PWM generators. Another popular modulation scheme is known as "space vector modulation," which requires six center-symmetric PWM outputs to drive an output bridge. Unfortunately, chips capable of such space vector modulation also tend to be more expensive than motor control chips having fewer than three PWM generators.
Thus, a need exists for providing three-phase sine waves from a microcontroller having less than three PWM generators. The present invention addresses these, and other, shortcomings associated with the prior art.